Geographical information systems (GIS) store, retrieve and display topological information. The topological information is obtained from a topology which is a topographic study of a geographic region. The topographic study may be a map having features of the geographic region including rivers, lakes, etc. and canals, bridges, roads, etc. GIS systems can be used in conjunction with GPS satellites to display the current location of a mobile GPS receiver along with the topological information on a display screen.
It would be desirable to display the current location of the GPS receiver along with the topological information on a display screen in real time for mission critical applications. The requirement for real time is that in certain situations a driver or pilot needs to make decisions and perform course corrections within a certain amount of time or it would be too late and the vehicle, airplane or ship would crash. In the GIS environment, real time means situations where there is a need to be able to draw a map in five seconds or less which includes the current position of the mobile GPS receiver. GIS systems known to the inventor are not capable of drawing a map which includes the current position of the mobile GPS receiver in five seconds or less.
Three problems make drawing a map which includes the current position of the mobile GPS receiver in five seconds or less difficult. The first problem concerns the display by GIS systems of topological regions in real time. The solution to this problem is addressed in my copending patent applications entitled "OBJECT-BASED GEOGRAPHICAL INFORMATION SYSTEM" and "TILING OF OBJECT-BASE D GEOGRAPHICAL INFORMATION SYSTEM" both filed on even date herewith and assigned to the instant assignee and both of which are hereby incorporated by reference in their entirety into this specification.
The second problem, scrolling a topological map displayed on a screen display in real time, is addressed in the previously mentioned "TILING OF OBJECT-BASED GEOGRAPHICAL INFORMATION SYSTEM" and is further addressed in this patent application.
The third problem concerns the precision and accuracy of the signals received from GPS satellites. This problem is addressed in this patent application. By way of explanation, the following background is provided regarding GPS satellite systems. Since the early 1980's the Global Positioning System (GPS) satellite system has been utilized. This system will eventually comprise a large number of satellites in orbits approximately 11,000 miles above the earth's surface inclined about 55.degree. from the equatorial plane. The satellites are not at a constant position but have a twelve hour orbit. At any point on the earth a ground based receiver can normally receive signals from at least four GPS satellites. A basic explanation of GPS and its use in surveying is given in Hurn. "GPS, A Guide to the Next Utility," Trimble Navigation, 1989, incorporated herein by reference thereto.
Each GPS satellite transmits signals which contain information that enables distance measurement to be made by measuring the transit time of a pseudo-random number (PRN) code from a satellite to a GPS receiver. The PRN code is a very faint signal that hardly registers above the earth's natural background noise; however, this signal can be received by an antenna only inches in size. Decoding of these signals is accomplished in known fashion by sampling the PRN code and correlating the code with a replica code generated by a GPS receiver thus permitting the PRN code to be picked out of the earth's background noise.
Two types of services produce signals from the GPS satellites. First, the Precise Position Service (PPS) produces for the military the most accurate dynamic positioning possible utilizing the GPS system, based upon the dual frequency Precise or Protected code known as the P code. Users must have an encryption code in order to access the P code which is not generally available to the public. Second, the Standard Positioning Service (SPS) produces the publicly accessible civilian positioning accuracy obtained by utilizing the single frequency "Clear Acquisition" (C/A) code. The Department of Defense has the ability to degrade the accuracy of the C/A code utilizing "Selective Availability" (S/A) or by artificially creating clock and other errors, to prevent hostile military forces from navigating accurately utilizing the C/A code. These errors include low frequency noise and high frequency noise.
Computation of positional coordinates utilizing GPS signal data may be simply accomplished by receiving the PRN code and recording the received time as measured by the receiver's clock. Relative clock offsets may be taken into account and the difference between a signal's departure time and arrival time is the total travel time. The distance from a GPS satellite to the receiver's position may then be approximated by multiplying the speed of light times the total travel time. In this manner, if time is known, a position may be determined using a minimum of three satellite signals. The calculated position can be at only one of two points at which three spheres around the three GPS satellites intersect. For a position known to be on earth, one of the points will generally be not possible (somewhere in space) so three satellites are generally enough to pinpoint a location. If precise time is not known then information from a fourth satellite will be necessary.
Thus, those skilled in the art will appreciate that GPS satellite broadcast systems permit PRN code tracking and analysis which permits positional fixes of high accuracy, but low precision due to refraction, clock errors, noise, time errors and ephemeris errors.
Upon reference to the foregoing those skilled in the art will appreciate that a need exists for a method and system which can draw a map in five seconds or less which includes the current position of the mobile receiver which may be a GPS receiver. A need exists for a method and system usable with a positioning system, such as GPS for displaying a topology and the location of a mobile receiver on a screen display of a GIS system and for updating the display of the topology and location of the mobile receiver on the screen display in real time as the mobile receiver changes locations. A need also exists for accurately determining the location of a mobile receiver by resolving the inaccuracy of the signals received from a positioning system, such as the GPS satellite system, using a predictive algorithm using a GIS system.